The present invention relates to liquid level sensing and, in particular, to sensing liquid levels in refillable reservoirs, such as sensing oil levels in gas turbine engines. Gas turbine engines include lubrication systems that use a lubricating liquid, such as oil, to cool and lubricate components such as gears and bearings. Such systems typically use oil during operation such that the quantity of oil in the lubrication system is diminished over time. Thus, lubrication systems typically include one or more sensors to determine the quantity of oil in the system. In addition, some systems incorporate an oil replenishment system that performs a process that refills a sump or other reservoir. Overfilling a sump can cause mechanical damage to engine components.
It can be difficult, however, to accurately determine the quantity of oil in a lubrication system at a given time. For example, one type of gas turbine engine is an auxiliary power unit (APU) for use on an aircraft. Varying oil temperature causes the actual volume of oil to vary, making it difficult to obtain useful sensor measurements. In addition, sloshing of oil during aircraft movement or while the engine is operating increases the difficulty of obtaining useful measurements. Consequently, oil levels measured during flight or while the engine is operating can be inaccurate.
Reading of sensors, such as capacitive sensors, can be used to provide relatively accurate readings while the oil environment is stable; however, such capacitance sensors can be relatively large, which can be problematic for use in relatively small sumps or other small reservoirs. In addition, these capacitance sensors require interfaces with noise shielding and compensative wires that add to system weight. Other available systems can also have limited effectiveness in small reservoirs, and can be complicated, expensive, relatively heavy, and/or unreliable.